Originally Posted by MotorradMike
Since MJ is off riding his new bike, I'm going to say what I think def.
He hand tapped that hole using a pin in the chuck to center the tap using the dimple in the top of the tap. That's what I was thinking, as well.
A little down force is all that is needed to get it started straight.
Once you get it started straight, it tends to stay that way. I'd be careful almost all the way through on this job. Fortunately, this was not a blind hole.
I usually chuck the tap and power it in part way, then stop and go manual after that because I don't have a tapping head yet.
I've tapped a pile of holes this way but never anything that big in that critical an application.
Although, with all due respect, he had nothing to lose at that point.
I bet the case would machine like butter, however, I don't know how it would be after he welded it. Some alloys are a bit brittle after welding.
I'd also like to know.
As a prototype machinst (~1970), I used to tap holes from 2-56 up to 1 inch in alloy and steel plate using a Bridgeport Vertical Mill (manual, no NC). I too chucked the tap and allowed the mill to center and rotate the tap but allowed the threads to feed the tap. My thinking was once the tap was started, it was best to keep constant pressure on the tap to preclude breaking the tap (which I did on several occasions).
Also, this hands off approach allowed me to apply cutting fluid and compressed air as the tap cut threads. Those were fun days when I learned a lot about metallurgy and just how tough some metals were and were not.
Alloys were always challenging for me. Most of my work was in alloy. Steel was more predictable to cut. I've never tried much welding. I had a friend who could weld steel and alloy beer cans together without blowing holes in the work...very steady hands and lots of patience.